The number of women ages 40 to 44 who remain childless has doubled in a generation, the U.S. Census reported Monday.

In June 2006, 20% of women in that age group remained childless. Thirty years ago it was 10%.

"A lot of women are having no children," Jane Lawler Dye, the author of the report, says. "Also, the women who are having children are having fewer children."

Birth rates for non-Hispanic white women, at 1.9 per woman, continue to remain lower than the rate that would be needed to replace the population (2.1 per woman), according to the report that details fertility rates in 2006 for women 15 to 44.

The report does not delve into the reasons behind the rates, but "a lot of it is delayed marriage and women getting started a little bit later in life," says David Hacker, assistant professor of history at Binghamton University, State University of New York.

The declining birth rate for older women "shows that patterns of family formation have obviously changed," says Carl Haub, a demographer at the non-governmental Population Reference Bureau in Washington, D.C. "There are significant numbers of women in the U.S. who would choose a career over having a child — married or unmarried."

The report also showed that the birthrate for Hispanic women is the highest among all ethnicities at 2.3 children each.

"The Hispanic population is growing," Dye says. "The black population is staying about the same, and the white and Asian populations are declining. They're not replacing themselves. So the population is growing more diverse."

Other trends:

• Utah, Nebraska and Idaho have the highest fertility rates; New Hampshire, Vermont and Rhode Island have the lowest.

• 36% of women who had children were not married.

• 57% of women with a recent birth were in the labor force.

• The highest fertility levels occurred among women with a graduate or professional degree.

"What that shows is that women are finishing their degrees and having a baby right away," Dye says. "Overall, women with higher education levels have lower levels of lifetime fertility."

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